The book entitled Heavy Book, which makes its first appearance in Comic 135, is, in itself, a heavy book. One would suppose it might also be about heavy books, and by judging its bulk, it is a heavy book. A heavy book about heavy books, perhaps? The purpose it serves is that of its weight, not its contents. Sounds like a boring read anyway.
As we find in Comic 137, the heavy book contains a page about anvils, and quite possibly speaks of other heavy things as well. Which might or might not also include heavy books, as speculated above.